1. Field of the Invention
The present invention relates to an image sensor device in which are arranged one- or two-dimensionally photoelectric converting elements (hereinafter called photosensor cells) each having a capacitor on the control electrode area of a semiconductor transistor.
2. Description of the Prior Art
Recent developments in image sensor devices have been principally conducted on devices of the CCD type and the MOS type.
The CCD image sensor device is based on the principle that potential wells are formed under MOS capacitor electrodes for accumulating electric charges generated by the entering light, and, in signal reading, said potential wells are moved in succession by pulses supplied to the electrodes, thereby transferring the accumulated charges to an output amplifier. On the other hand, the MOS image sensor device is based on the principle that electric charges generated by the entering light are accumulated in photodiodes constituting a light-receiving area and each comprising a p-n junction, and, in signal reading, MOS switching transistors respectively connected to said photodiodes are turned on in succession to transfer the accumulated charges to an output amplifier.
However, such conventional image sensor devices have been associated with the following drawbacks which will be a major obstacle in achieving higher sensitivity and higher resolving power in the future.
More specifically, the CCD image sensor devices have the drawbacks that (1) the MOS output amplifier, If the formed on the same chip, tends to generate, from the interface between silicon and a silicon oxide layer, l/f noises which are easily noticeable on the image, (2) the maximum charge that can be accumulated in a potential well is reduced and the dynamic range becomes smaller if the number of cells is increased with a higher density in order to achieve a higher resolving power, and (3) the sequential charge transfer is interrupted if only one cell is defective.
On the other hand, the MOS image sensor devices have drawbacks that (1) a large drop in the signal voltage occurs upon signal reading since a wiring capacitance is connected to each photodiode, (2) a large wiring capacitance tends to generate random noises, and (3) noises of a fixed pattern are present because of fluctuation in the parasitical capacitances of the MOS switching transistors. Thus, for example, in a two-dimensional solid-state image sensor device, image taking under a low illumination becomes difficult, and size reduction of the cells for achieving a higher density deteriorates the S/N ratio, since the wiring capacitance is not significantly reduced while the accumulated charge becomes smaller.
In this manner the CCD and MOS image sensor devices are associated with fundamental problems in achieving a higher resolving power. On the other hand, there has been proposed a new semiconductor image sensor device (European Patent Application Laid-open No. 0132076) in which electric charges generated by the entering light are accumulated in control electrodes, for example bases of bipolar transistors or gates of static induction transistors (SIT) or MOS transistors, and the accumulated charges are amplified at signal reading by the amplifying function of the cells. This structure can provide a high output, a wide dynamic range, a low noise level and a non-destructive signal reading, thus leading to a possibility for a higher resolving power.
However there is a certain limit in the resolving power since this structure, if employed in a two-dimensional image sensor device, is fundamentally based on x-y addressing, and each cell is basically composed of a conventional MOS cell combined with an amplifying element such as a bipolar transistor or a SIT transistor. Also in case a large number of cells are arranged for achieving a high resolving power, a high-speed function is difficult to achieve because shift registers for reading the output signals become complicated. Besides, the S/N ration becomes lower though the output impedance becomes higher.
Also a one-dimensional image sensor device is associated with certain drawbacks which will be explained in the following in relation to the accompanying drawings.
FIG. 1 is a circuit diagram of a conventional one-dementional image sensor device containing an array of plural photosensor cells, in which electric charges are accumulated in capacitors electrically connected to control electrode areas.
Referring to FIG. 1, said photosensor cells 101 are linearly arranged, and output terminals thereof are connected, through MOS transistors 5.sub.1 -5.sub.n, to a common output line 20. Said common output line 20 is connected to an output terminal 22 through an output amplifier 21, and is also grounded through a line-refreshing MOS transistor 24. The gates of the MOS transistors 5.sub.1 -5.sub.n are respectively connected to terminals 7.sub.1 -7.sub.n of a logic circuit 19.
In such a conventional image sensor device, the optical information signals of the photosensor cells 101 are serially released through the common output line 20 and the output amplifier 21 by serial activation of the MOS transistors 5.sub.1 -5.sub.n.
The time required for signal reading from a photosensor cell 101 is proportional to the floating capacity of the common output line 20. Consequently, the total time required for signal reading rapidly increases with an increase in the number n of the photosensor cells 101.
In this manner, in the conventional structure, the high-speed signal reading operation becomes difficult to achieve, when the number n of the photosensor cells is increased in order to achieve a high resolving power.
Furthermore, in the above-explained photosensor cell 101 in which the electric charge generated by photoexcitation is accumulated in the control electrode, the accumulated charge has to be dissipated for refreshing, after the signal reading is completed. Since such refreshing operation has been conducted at a time for all the photosensor cells 101, so that, particularly in a two-dimensional image sensor device, the time from said refreshing operation to the start of signal reading is different for all photosensor cells 101, and for this reason it has been difficult to achieve a high-speed operation and a uniform photoelectric converting property.